It is known that most of the aircrafts, in particular civil transport airplanes, are equipped with an automatic pilot for controlling the trajectory according to set points being preset by the crew. Such an automatic pilot has the major advantage of decreasing the work load on the crew by relieving them from a part of the navigation task, during most of the flight phases. It is further known that connecting and disconnecting the automatic pilot are carried out by a voluntary action of the crew.
However, under some particular circumstances (breakdown, disturbances, etc.), the automatic pilot is caused to be disconnected by itself.
Regardless of the cause of the automatic pilot disconnection, the crew is informed about it through one or more audible alarms, as well as through visual changes on the piloting interfaces.
But, in spite of such visual and audible alerting systems, many cases are reported where the crews do not become aware of the automatic pilot disconnection, so either:                the crew must face high or particular stressing work load situations which may alter their attention abilities and cause them to neglect alarms, such as those of automatic pilot disconnection;        or a lack of comprehension within the crew in distributing the piloting task occurs;        or the alarm for the automatic pilot disconnection is masked by a simultaneous alarm of higher priority (for example an alarm indicating that the aircraft is coming out of its flight envelope).        
Besides, the existence of flight control laws, which stabilise the aircraft trajectory, can contribute to give the illusion that the automatic pilot is still operating and not cause the crew to check the condition of the automatic pilot.
Of course, it is particularly dangerous for the aircraft not to be piloted either by the crew, or by the automatic pilot.
Thus, the object of the present invention is to specifically detect such a situation and to inform the crew about it.